Open-loop transimpedance amplifier for infrared diodes

ABSTRACT

A microcontroller integrated circuit includes an open-loop transimpedance amplifier (OLTA). An input lead of the OLTA is a terminal of the microcontroller. The cathode of a photodiode is connected to VDD and the anode is connected to the terminal. The OLTA maintains the photodiode in a strongly reverse-biased condition, thereby keeping diode capacitance low and facilitating rapid circuit response. The input of the OLTA involves a diode-connected field effect transistor that provides a low impedance. This low impedance decreases as the diode current increases, thus providing effective clamping of the voltage on the terminal. By this clamping, the amount of photodiode capacitance discharging necessary when transitioning from a high input current condition to a low input current condition is reduced, thereby further improving amplifier response time. The OLTA is small and consumes less than thirty microamperes and functions to mirror photodiode current and compare to a predetermined level.

TECHNICAL FIELD

The described embodiments relate to infrared receiver circuits, and moreparticularly to an infrared receiver circuit that is fully integratedonto a microcontroller integrated circuit within a learning remotecontrol device.

BACKGROUND INFORMATION

Manufacturers of electronic consumer devices (for example, televisions,radio tuners, home theatre and entertainment systems, digital video disc(DVD) players, video cassette recorders (VCR), compact disc (CD)players, set-top cable television boxes, set-top satellite boxes, videogame controllers, home appliances, etc.) typically supply an infraredremote control device along with each electronic consumer device. Suchan infrared remote control device is often a handheld battery-powereddevice with a set of keys and an infrared (IR) transmitter. The remotecontrol device can control the associated electronic consumer device bysending an appropriate infrared operational signal to the electronicconsumer device. The operational signal carries a key code. Each suchkey code corresponds to an associated function of the selectedelectronic consumer device. Such functions may include power on/off,volume up, volume down, play, stop, select, channel advance, channelback, etc.

If, for example, an individual in the home wishes to increase the volumeof a television, then the individual presses the “volume up” key on theremote control device for the television. Circuitry in the remotecontrol device detects the key press condition, accesses appropriate keycode and modulation information stored in the remote control device, anduses the key code and modulation information to generate an appropriatecontrol signal that is used to drive an infrared light emitting diode(LED). This control signal causes the LED to transmit the infraredoperational signal to an infrared receiver in the television. The keycode is carried by the operational signal. The infrared receiver in thetelevision receives the infrared operational signal, detects the keycode, and takes an action that is appropriate for the key code. In thepresent example where the “volume up” key was pressed, the appropriateaction is to increase the audio output volume of the television.

A typical user in the home may have many different electronic consumerdevices that are to be controlled. A user may, for example, have adigital video disc (DVD) player and a television. To view a movie on aDVD, the user may have to power on and control the DVD with a firstremote control device that issues operational signals that the DVDplayer responds to. In addition, the user may have to power on andcontrol the television with a second remote control device that issuesoperational signals that the television responds to. It is desired toreduce the number of remote control devices in this situation to onesuch that a single remote control device is usable to control bothelectronic consumer devices (the DVD player and the television).

A type of remote control device referred to as a “learning remotecontrol device” may be employed to replace both remote control devicesin the exemplary situation described above. The learning remote controldevice has infrared receiver circuitry as well as conventional infraredtransmitter circuitry. The learning remote control device is placed suchthat the infrared receiver of the learning remote control device canreceive infrared operational signals transmitted from one of the remotecontrol devices to be replaced. A key on the remote control device to bereplaced is then pressed. The infrared receiver in the learning remotereceives the infrared operational signal and stores information aboutthe operational signal such that the learning remote control device canlater regenerate the operational signal using the infrared transmittercircuitry of the learning remote control device. This process ofdetecting and storing information that is usable to regenerate anoperational signal is called “learning”.

After the learning remote control device has learned how to regenerateoperational signals output from one remote control device to bereplaced, the learning remote control device learns how to regenerateoperational signals output from another remote control device to bereplaced. Thereafter, the user can use the learning remote controldevice to emulate either the first or the second remote control device.The user controls which of the two remote control devices will beemulated by changing a mode of the learning remote control device. Thelearning remote control device is therefore now usable to control theboth electronic consumer devices in the home, thereby replacing themultiple manufacturer-supplied remote control devices.

The circuitry in the learning remote control device generally includes amicrocontroller integrated circuit, an infrared photodiode, and aninfrared receiver circuit. The infrared receiver circuit receives asignal from the infrared photodiode and outputs a digital output signalonto a serial input terminal of the microcontroller integrated circuit.The infrared receiver circuit is typically a fairly expensive circuitthat consumes a substantial amount of power when it is functioning.Traditional techniques involve realizing the infrared receiver indiscrete circuit components (including discrete resistors and/orcapacitors) located outside the microcontroller integrated circuit. Inone example, the infrared receiver circuit includes multiple operationalamplifier gain stages, each including a feedback loop having resistors.The operational amplifier circuit consumes three hundred microamperes ormore when it is receiving an infrared signal from an infraredphotodiode. In another example, a cascode bipolar transistor amplifiercircuit involves multiple resistors and a capacitor in a biasingnetwork. If either of these traditional infrared receiver circuits wereto be integrated into the microcontroller integrated circuit, then theresistors and capacitors and/or the complex operational amplifiercircuitry would consume an undesirably large amount of die area, therebyincreasing the cost of the microcontroller integrated circuit.Accordingly, integrating an infrared receiver circuit onto amicrocontroller integrated circuit that is to see general usage innon-learning remote control devices can be recognized to be unacceptablyexpensive. Moreover, the hundreds of microamperes of power consumed bysuch a traditional infrared receiver would be undesirable. An improvedmicrocontroller integrated circuit that has an improved,fully-integrated infrared receiver is desired.

SUMMARY

It is recognized that the high sensitivity afforded by conventionalphotocurrent operational amplifier circuits is unnecessary in a learningremote control device where the remote control device from which aninfrared operational signal is to be learned is generally placed inclose proximity to the photodiode of the learning remote control device.In addition, the photodiode in a learning remote control device involvesa parasitic capacitance. Ordinary amplifier circuits used in learningremote control devices to amplify photocurrents can have significantinput impedances can therefore be slow in discharging the photodiode'sparasitic capacitance, thereby contributing to slow response times.

A novel microcontroller integrated circuit is disclosed that includes anovel open-loop transimpedance amplifier (OLTA). An input node of theOLTA is an input terminal of the microcontroller. In one embodiment,when the microcontroller is used in a learning remote control device,the cathode of a photodiode external to the microcontroller integratedcircuit is connected to a supply voltage VDD and the anode of thephotodiode is connected to the input terminal. The photodiode supplies aphotocurrent onto the input terminal of the microcontroller (andtherefore onto the input node of the OLTA). If the input photocurrent isless than a “trip point input current” (for example, due to darkconditions), then the OLTA forces a digital output signal DATA on anOLTA output lead to a digital logic high value. If the inputphotocurrent is more than the “trip point input current” (for example,due to the infrared photodiode receiving infrared radiation from anoperational signal), then the OLTA forces the signal DATA to a digitallogic low value.

The OLTA includes a diode-connected N-channel transistor circuit as aninput stage. The diode-connected N-channel transistor circuit biases thevoltage on the input terminal under dark conditions (photodiode notactivated by light) at approximately one N-channel transistor Vt(threshold voltage) above ground potential. Because the cathode of thephotodiode is coupled to VDD and the anode is coupled to the inputterminal, the photodiode is biased in a strongly reverse-biasedcondition. Keeping the photodiode strongly reverse-biased minimizes theparasitic capacitance of the photodiode and thereby facilitates fastresponse times of the OLTA.

The diode-connected N-channel transistor of the input stage of the OLTAalso causes the input terminal of the microcontroller to have a lowinput impedance. In one example, the input impedance is forty ohms orless when input currents of eight milliamperes of more are beingreceived into the input terminal.

In another novel aspect, the input impedance of the input terminaldecreases as the diode current flowing into the terminal increases, thusproviding effective clamping of the voltage on the input terminal of themicrocontroller. The input voltage on the input terminal is clamped forphotocurrents over a wide dynamic range (for example, from zerophotocurrent to approximately 10 milliamperes of photocurrent). By thisclamping, less discharging of photodiode parasitic capacitance whentransitioning from a high input current condition to a zero inputcurrent condition is required, thereby further improving amplifierresponse time. The OLTA involves no feedback loop, no large resistorsand/or capacitors and/or operational amplifiers, and therefore can beintegrated into a small die area. In one example, the OLTA: 1) isrealized in 20,000 square microns of integrated circuit die area, and 2)consumes less than 30 microamperes of supply current when receiving aphotodiode input current signal having an amplitude of greater than 100microamperes and having a signal rate from zero of up to at least fivehundred kHz. The OLTA functions over a wide supply voltage range and itrequires no voltage references or power supply voltage dividers.

Further details and embodiments are described in the detaileddescription below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components,illustrate embodiments of the invention.

FIG. 1 is a diagram of a learning remote control device that includes aphotodiode 3 and a novel microcontroller integrated circuit (themicrocontroller integrated circuit is contained in the plastic enclosureof the remote control device). The novel microcontroller integratedcircuit includes a novel OLTA (open-loop transimpedance amplifier) inaccordance with one novel aspect.

FIG. 2 is a circuit diagram showing the photodiode 3 of FIG. 1 coupledto the novel OLTA 101 within microcontroller integrated circuit 100.

FIG. 3 is a waveform diagram showing operation of the OLTA of FIGS. 1and 2 when a photodiode input current having a digital amplitude of 100microamperes flows into the OLTA.

FIG. 4 is a waveform diagram showing operation of the OLTA of FIGS. 1and 2 when a photodiode input current having a digital amplitude of 10milliamperes flows into the OLTA.

FIG. 5 is a diagram of an IV (current-to-voltage) input characteristicof the OLTA of FIGS. 1 and 2.

FIG. 6 is a flowchart of a method in accordance with one novel method.

FIG. 7 is a circuit diagram of another embodiment.

DETAILED DESCRIPTION

FIG. 1 is a diagram of a learning remote control device 1 that includesa novel microcontroller integrated circuit. Learning remote controldevice 1 is usable to receive and “learn” an infrared operational signal2 transmitted from another infrared remote control device. Operationalsignal 2 is received by an infrared photodiode 3. The novelmicrocontroller integrated circuit (not shown in FIG. 1) is disposedwithin the plastic housing of the learning remote control device 1. Themicrocontroller integrated circuit is coupled to infrared photodiode 3as explained in further detail below. Learning remote control device 1also includes infrared transmitter circuitry and an infrared lightemitting diode (LED) 4 for transmitting an infrared operational signal5. An infrared operational signal that is “learned” by learning remotecontrol device 1 in association with a key 6 can be regenerated andtransmitted from learning remote control device 1 at a later time bypressing key 6 when learning remote control device 1 is in anappropriate mode of operation.

An infrared operational signal can be “learned” in the sense that timinginformation on when the received operational signal transitions low andhigh is stored and then the microcontroller later uses this timinginformation to regenerate a facsimile of the received operationalsignal. Alternatively, an infrared operational signal can be “learned”by detecting when the received operational signal transitions low andhigh and then by using this information to search a group codesets toidentify which codeset contains information for generating anoperational signal having similar high and low transition timing. Oncethe proper codeset is identified, then the microcontroller uses a keycode and modulation information and other information in the codeset toregenerate the operational signal.

FIG. 2 is a simplified circuit diagram showing the incoming infraredoperational signal 2, infrared photodiode 3, and a novel microcontrollerintegrated circuit 100 that includes a novel open-loop transimpedanceamplifier (OLTA) 101. The vertical dashed line 102 in FIG. 2 representsa boundary of microcontroller integrated circuit 100. The cathode ofdiode 3 is coupled to a power supply (VDD) terminal 103 ofmicrocontroller integrated circuit 100. The anode of diode 3 is coupledto photodiode current input terminal (PD) 104. An input lead 105 of OLTA101 is directly coupled to an input terminal (PD) 104 of themicrocontroller and to an OLTA input node N1. In the illustratedspecific embodiment, an output lead 106 of OLTA 101 is coupled to a datainput lead of a timer 107. A digital processor 108 reads and controlstimer 107, thereby obtaining timing information about when the variousedges of a digital signal passing into the timer 107 occurred. Timer 107and processor 108 are specific to the particular embodiment depictedhere. Other suitable circuitry for interfacing a digital processor of amicrocontroller to a photocurrent amplifier circuit that outputs adigital signal can be employed. In the example of FIG. 2,microcontroller integrated circuit 100 and photodiode 3 are powered bytwo AA batteries 109 and 110. The proper operating range of the supplyvoltage VDD between supply terminal 103 and ground terminal 145 is from1.8 volts to 3.6 volts.

Input terminal 104 has an input impedance less than forty ohms when aninput current 111 of eight milliamperes or more is flowing into inputterminal 104. OLTA 101 has a non-linear IV (current-to-voltage)characteristic. As the input current 111 increases, the voltage on inputnode N1 increases proportional to the square root of the input currentincrease. Accordingly, the input impedance decreases as input current111 (in an input current operating range) increases. The input currentoperating range in the example of FIG. 2 extends from zero microamperes(when no infrared signal is being received onto diode 3) toapproximately ten milliamperes (when a strong infrared signal is beingreceived onto diode 3). Because the input impedance decreases morerapidly than the input current increases over this input current range,the voltage on PD input terminal 104 is effectively clamped to an inputvoltage range of less than approximately 0.7 volts for levels of inputcurrent 111 in the normal input current operating range (from zero to 10milliamperes) of the circuit. If input current 111 is less than a “trippoint input current” of approximately 32.0 microamperes, then a digitallogic zero signal is output onto output lead 106. If input current 111is more than the “trip point input current”, then a digital logic highvalue is output onto output lead 104. This 32.0 microampere trip pointinput current level is referred to here as the “sensitivity” of theamplifier. The higher the trip point input current level, the less“sensitive” the amplifier is said to be. In one advantageous aspect,OLTA 101 has a sensitivity less than 20 microamperes (the “trip pointinput current” is higher than 20 microamperes) but nonetheless functionsacceptably in the application of “learning” an infrared operationalsignal emitted from a remote control device.

OLTA 101 receives a 0.67 microampere bias current (IIN) 112 on a biascurrent input lead 113 from elsewhere on the microcontroller integratedcircuit. If OLTA 101 is enabled by virtue of an enable signal (EN) onenable input lead 114 being a digital logic high, then the pass-gateformed by transistors 115 and 116 is conductive. The bias current 112that flows through N-channel transistor 117 is mirrored and multipliedby twelve by the current mirror formed by N-channel transistors 117 and118. A mirrored current 119 of approximately 8.0 microamperes thereforeflows through N-channel transistor 118. This mirrored current 119 alsoflows through P-channel transistor 120. The 8.0 microamperes of current119 flowing through P-channel transistor 120 is in turn mirrored byP-channel mirroring transistors 121-124 into four corresponding mirroredcurrents 125-128. The relative sizes of P-channel transistors 121-124 toP-channel transistor 120 determines the relative magnitudes of thecurrents 125-128. In the example of FIG. 2, the four currents 125-128are 8.0 microamperes, 4.0 microamperes, 4.0 microamperes, and 8.0microamperes, respectively.

Dark Condition Operation:

In operation, when there is a dark condition (substantially no infraredradiation is being received by diode 3) and diode photocurrent 111 isapproximately zero, then substantially no current is flowing into inputterminal 104 and to node N1. The 8.0 microamperes of current 125 fromP-channel transistor 121 therefore flows from drain to source through aso-called “diode-connected N-channel transistor” 129.

Diode-connected N-channel transistor 129 is not a real diode in thesense that a real diode or a diode-connected bipolar transistor hasadjacent oppositely doped semiconductor regions and has an exponentialcurrent-to-voltage relationship for forward voltages across thejunction. Rather, diode-connected N-channel transistor 129 is anN-channel transistor whose drain is connected to its gate to form a twoterminal device. The gate-drain node is a first terminal. The sourcenode is a second terminal. In an N-channel transistor in the saturationregion of operation, the drain current is roughly proportional to thesquare to the gate-to-source voltage when a forward gate-to-sourcevoltage on the transistor is greater than a threshold voltage (Vth).Accordingly, if the drain and gate of an N-channel transistor areconnected as a so-called “diode-connected N-channel transistor”, and ifa forward current is made to flow from drain to source through thetransistor, then increases in the drain current will roughly beproportional to the square of the corresponding increases ingate-to-source voltage. Due to this operation, which is similar to theoperation of a real diode in some respects, an N-channel transistor usedin this way whose drain is connected to its gate is referred to here asa “diode-connected N-channel transistor”.

In OLTA 101 of FIG. 2, transistors 130 and 131 form a passgate. Thispassgate is conductive when the enable signal EN is a digital logichigh. The gate 132 of N-channel transistor 129 is therefore connected bythe passgate to the transistor's drain at node N1. The body of N-channeltransistor 129 is coupled to ground potential. Due to the action ofP-channel pullup transistor 121, the gate-to-source voltage acrossdiode-connected N-channel transistor 129 is approximately the thresholdvoltage (approximately 0.5 volts) of N-channel transistor 129. N-channeltransistor 129 therefore conducts the drain current supplied byP-channel transistor 121. Further increases in forward drain current 134(above the drain current at which transistor 129 begins to conduct)result in much smaller gate-to-source voltage increases. As set forthabove, in a diode-connected N-channel transistor the drain currentincreases proportionally to the square of the increase in gate-to-sourcevoltage. Diode-connected N-channel transistor 129 therefore functions tobias the voltage on input terminal 104 to approximately one thresholdvoltage (approximately 0.5 volts) above ground potential on groundconductor 146.

The 8.0 microamperes of current 134 flowing through input transistor 129due to P-channel transistor 121 is mirrored by a current mirror formedby N-channel transistors 129 and 133. Input transistor 129 has awidth/length ratio of 600/1 whereas mirroring transistor 133 has awidth/length ratio of 60/1. Mirrored current 135 flowing throughtransistor 133 is therefore one tenth as small as current 134. Current135 is therefore 0.8 microamperes. Because P-channel transistor 122 isto supply the 4.0 microampere current 135 into node N2 and becauseN-channel transistor 133 only sinks 0.8 microamperes of current 135 toground, the voltage on node N2 is pulled high. The voltage on node N2increases until the drain of P-channel transistor 122 increases to thepoint that transistor 122 no longer acts as a current mirror.

When the voltage on node N2 is adequately high, N-channel transistor 136becomes conductive. N-channel transistor 136 can sink more than 4microamperes of current to ground potential, so transistor 136 beingmade, conductive overdrives P-channel transistor 123 and pulls thevoltage on node N3 to a digital logic low. When the voltage on node N3falls, N-channel transistor 137 is made non-conductive. P-channeltransistor 124 therefore pulls the voltage on node N4 upward until thedrain of P-channel transistor 124 increases to the point that transistor124 no longer acts as a current mirror. When the enable signal EN is adigital logic high, P-channel transistor 138 is non-conductive anddigital logic high values are present on the upper input leads of NANDgates 139 and 140. The digital logic high on node. N4 is thereforeinverted by NAND gate 139, is again inverted by NAND gate 140, and isinverted again by inverter 141. The resulting digital logic low value isoutput as digital signal DATA onto OLTA output lead 106.

Input Current Trip Point Operation:

If 32.0 microamperes of photocurrent 111 is flowing into PD inputterminal 102 and to node N1, then this photocurrent adds to the 8.0microamperes of current 125 such that current 134 flowing throughdiode-connected N-channel transistor 129 is 40.0 microamperes. Due tothe operation of the current mirror of transistors 129 and 133, mirroredcurrent 135 is one tenth of 40.0 microamperes or 4.0 microamperes. This4.0 microamperes of current 135 matches the 4.0 microamperes of current126 supplied by P-channel transistor 122. If current 135 is any greaterthan 4.0 microamperes, then the voltage on node N2 will be pulled downand OLTA 101 will force the output signal DATA to a digital logic highvalue. If current 135 is any smaller that 4.0 microamperes, then thevoltage on node N2 will be pulled up and OLTA 101 will force the outputsignal DATA to a digital logic low value. Accordingly, 32.0 microamperesof input current 111 represents the input current trip point betweenoutputting a digital logic low value of signal DATA (representing a darkcondition) onto OLTA output lead 106 and outputting a digital logic highvalue of signal DATA (representing an infrared radiation detectedcondition) onto OLTA output lead 106.

Infrared Radiation Detected Condition:

When adequate infrared radiation is detected by diode 3, then inputphotocurrent 111 has a magnitude that exceeds 32.0 microamperes.Consider a situation where the input photocurrent is 100 microamperes.108 microamperes are therefore flowing into node N1, and current 134flowing through diode-connected N-channel transistor is 108.0microamperes. Mirrored current 135 is therefore 10.8 microamperes.N-channel transistor 133 overdrives the current mirror of transistor122. Node N2 is therefore pulled to ground potential. The low voltage onthe gate of N-channel transistor 136 makes N-channel transistor 136non-conductive such that P-channel transistor 123 pulls the voltage onnode N3 high. The voltage on node N3 stops rising when P-channeltransistor 119 is saturated and is no longer acting as a current mirror.The high voltage on node N3 makes N-channel transistor 137 adequatelyconductive that it overdrives the current mirror of P-channel transistor124. The voltage on node N4 is therefore below the switching voltage ofNAND gate 139. The digital logic low on the lower input lead of NANDgate 139 is inverted three times by NAND gate 139, NAND gate 140 andinverter 141 such that a digital logic high value of the signal DATA isoutput onto output lead 106.

FIG. 3 is a waveform diagram. Waveform 200 represents the voltage oninput terminal 104 when an input current 111 having a digital logic lowlevel of zero amperes and a digital logic high level of 100 microamperesis supplied onto OLTA input lead 105. Waveform 201 represents theresulting DATA signal output onto OLTA output lead 106.

When diode 3 is highly illuminated with infrared radiation, diode 3 canoutput a significant amount of current. In the present example, diode 3can output ten milliamperes or more. The photodiode has a parasiticcapacitance between its cathode and anode terminals. If a traditionalinfrared photocurrent amplifier circuit having a high input impedancewere directly connected to the cathode terminal of diode 3, and if theinfrared amplifier circuit were a voltage detecting device, then whendiode 3 suddenly switched from a highly illuminated condition to a darkcondition, the voltage on the input node of the amplifier would remainon the input node due to the capacitance of the diode 3 itself. Only asthe voltage on the input node decays to the voltage trip point of theinfrared receiver circuit would the amplifier receiver circuit detectthe dark condition. The result would be a slow response time of theinfrared amplifier receiver circuit when going from an illuminatedcondition to a dark condition. The slow response time is made worseunder conditions of very high illumination. Due to the substantial inputimpedance of the infrared receiver circuit, the high diode currentcorresponding to the very high illumination would cause the voltage onthe input of the receiver circuit to rise a significant amount. When thediode suddenly stops outputting photocurrent due to a dark condition,the high-voltage on the input of the receiver circuit would have todischarged down to the trip point of the receiver circuit before thereceiver circuit could detect a dark condition. The higher the voltageon the input node, the longer it would take to discharge the diodecapacitance on the input of the receiver circuit.

PD input terminal 104, however, has a low input impedance. When diodecurrent 111 falls rapidly upon a transition from an illuminated diodecondition to a dark diode condition, the low input impedance of terminal104 facilitates discharging of the diode capacitance through the inputterminal 104 and thereby decreases amplifier response time. In theexample of FIG. 2, the input impedance of input terminal 104 is lessthan forty ohms when an input current 111 of eight milliamperes or moreis flowing into input terminal 104.

Not only is response time decreased due to the low input impedance ofinput terminal 104, but OLTA 101 also decreases response time bypreventing large voltages on input terminal 104 during high diodecurrent conditions. As set forth above, transistor 129 is adiode-transistor N-channel transistor having a non-linear IV (current tovoltage) characteristic. Due to this IV characteristic, a large increasein current 111 gives rise to only a small increase in the voltage onnode N1. At a first approximation, the current 111 increases as thesquare of the voltage on node N1 (a diode-connected field effecttransistor is sometimes referred to as a “square-law device”).Accordingly, the voltage on node N1 does not increase linearly even withlarge magnitudes of diode current 111. In the example of FIG. 2 wherethe highest diode input current 111 under normal operating conditions isapproximately 10 milliamperes, the voltage on input terminal 104 (and oninput node N1) is clamped to be in a range between 0.5 volts andapproximately 1.2 volts. Because the voltage on the input terminal isclamped under high diode current conditions, only a relatively lowvoltage on the input terminal 104 need be discharged in order for OLTA101 to detect a dark condition. As illustrated in FIG. 3, the responsetime from when input current 111 begins to decrease from its highcurrent level to its low current level until the OLTA digital outputsignal DATA begins to transition from a digital high to a digital low isapproximately 0.20 microseconds.

FIG. 4 is a waveform diagram that shows the voltage 300 on inputterminal 104 when input current signal 111 has a first current level ofzero amperes and a second current level of 10 milliamperes. Waveform 301represents the resulting digital signal DATA on output lead 106. WhenFIGS. 3 and 4 are compared, it is noted that: 1) a zero input currentcondition results in 0.5 volts on input terminal 104; 2) a 100microampere input current condition results in 0.63 volts on inputterminal 104; and 3) a 10 milliampere input current condition results in1.13 volts on input terminal 104. Accordingly, a digital input currentsignal having a 100 microampere digital amplitude on input terminal 104results in a 0.13 volt digital amplitude signal on input terminal 104. Adigital input current signal having a 10 milliampere (100 times higherthan 100 microamperes) digital amplitude on input terminal 104 resultsin a 0.63 volt digital amplitude signal on input terminal 104 (onlyabout 4.5 times higher than 0.13 volts). This non-linear input impedanceof input terminal 104 allows for faster recovery to the no signal level(i.e., dark condition) than a fixed resistive input impedance wouldallow for. As illustrated in FIG. 4, the response time from when inputcurrent 111 begins to decrease from its high current level (the 10milliampere level) until the OLTA digital output signal DATA begins totransition from its digital high level is approximately 0.25microseconds.

FIG. 5 is a diagram showing an IV characteristic of OLTA 101 of FIG. 2.OLTA 101 has a input current operating range from a minimum inputcurrent value of approximately zero milliamperes to a maximum inputcurrent value of approximately 10 milliamperes. Due to the non-linearcurrent-to-voltage characteristic of OLTA 101, the input voltage oninput terminal 104 is effectively clamped to less than 1.2 volts asillustrated. As the diode current 111 increases, the drain voltage ondiode-connected transistor 129 increases very little past the thresholdvoltage of transistor 129. Input transistor 129 clamps the voltage onnode N1 to a reasonable level even if tens of milliamps of photocurrentare flowing into terminal 104. This is important because transistor 129functions to pull the voltage on node N1 back down when the diodecurrent 111 falls at the beginning of a dark condition. Becausediode-connected transistor 129 is a square-law device, the voltage onnode N1 is pulled down faster than if a simple fixed resistance (that ispractical for an amplifier in this application) were used.

In one advantageous aspect, OLTA 101 involves no resistors or capacitorsthat if realized in integrated form would consume large amounts of diearea. OLTA 101 includes no bipolar transistors and is realized inintegrated form in 20,000 square microns in a standard 0.5 micron CMOSprocess. In contrast to traditional infrared diode receiver circuitsinvolving operational amplifiers, OLTA 101 involves no feedback loop. Inoperation, not all of current-mirroring P-channel transistors 121-124are sourcing the indicated currents because some of the nodes N1-N4 areat high voltage levels and the associated current sources are currentstarved. In contrast to a typical infrared diode operational amplifierreceiver circuit that may consume 300 microamperes or more, OLTA 101 ofFIG. 2 consumes less than 30 microamperes when receiving a 500 KHzdigital input current signal having a 100 microampere amplitude. Thereference here to “a 100 microampere amplitude” means the input signal,has a first current level that corresponds to zero milliamperes of inputcurrent and has a second current level that corresponds to 100microamperes of input current. Capacitors 142 and 143 are optional andmay be included in OLTA 101 to provide a measure of noise filtering suchthat short-lived changes in input current 111 do not pass through OLTA101 and appear on output lead 106. Capacitors 142 and 143 may, forexample, be 0.5 picofarad capacitors.

Disable:

When OLTA 101 is not being used to “learn” an operational signal, OLTA101 is disabled to stop OLTA from consuming power. Processor 108 may,for example, write to an OLTA enable bit in a control register. Thevalue of the OLTA enable bit determines the digital logic value of theenable signal EN on input lead 114. If the enable signal EN is a digitallogic low, then the passgate formed by transistors 115 and 116 isnon-conductive and N-channel transistor 144 is conductive. N-channeltransistor 144 being conductive couples node N5 to ground potential,thereby disabling the current mirror formed by transistors 117 and 118.Current 119 is cut to zero. In addition, the passgate formed bytransistors 130 and 131 is made non-conductive, thereby decoupling thegate 132 of transistor 129 from node N1. Rather than coupling the gate132 of transistor 129 to the drain of transistor 129, the gate 132 iscoupled through conductive transistor 140 to ground potential. Groundingthe gate 132 of transistor 129 makes transistor 129 non-conductive andcuts the currents 134 and 135 to zero, thereby conserving power. Whenthe enable signal EN is a digital low, P-channel transistor 138 isenabled, thereby coupling node N4 and the lower input lead of NAND gate139 to supply voltage VDD. The output signal DATA on output lead 106 istherefore maintained at a digital logic low value.

FIG. 6 is a flowchart of a method in accordance with one novel aspect.Initially (step 400), a photodiode is coupled to an input terminal of anopen-loop transimpedance amplifier (OLTA). In one example, thisphotodiode is photodiode 3 of FIG. 2 and the input terminal is inputterminal 104 of microcontroller integrated circuit 100. A photocurrentfrom the photodiode is received (step 401) onto a diode-connectedN-channel transistor within the OLTA. In one example, thisdiode-connected N-channel transistor is transistor 129 of FIG. 2.Operation of the diode-connected N-channel transistor effectively clampsthe voltage on the input terminal. In the example of FIG. 2, the voltageon input terminal 104 is clamped within a range of approximately 0.7volts (0.5 volts to 1.2 volts) for photocurrents in an inputphotocurrent operating range (from zero microamperes to tenmilliamperes). The diode-connected N-channel transistor causes the inputterminal to have a low input impedance that decreases with increases inthe photocurrent. A current flowing through the diode-connectedN-channel transistor is mirrored (step 402) to obtain a mirroredcurrent. In the example of FIG. 2, the current 134 flowing throughdiode-connected transistor 129 is mirrored by transistor 133 to producemirrored current 135. If the mirrored current is greater than apredetermined current (step 403), then the OLTA output signal DATA has afirst digital logic value (step 404). In the example of FIG. 2, ifmirrored current 135 is greater than the predetermined current level of4 microamperes flowing through P-channel transistor 122, then OLTAoutputs a digital logic high value onto output lead 106. If the mirroredcurrent is not greater than the predetermined current (step 403), thenthe OLTA output signal DATA has a second digital logic value (step 405).In the example of FIG. 2, if mirrored current 135 is not greater thanthe predetermined current level of 4 microamperes flowing throughP-channel transistor 122, then OLTA outputs a digital logic low valueonto output lead 106.

FIG. 7 is a circuit diagram of another embodiment. Rather than usingtransistor 133 to mirror the current 134 flowing through thediode-connected N-channel transistor 129 as in the circuit of FIG. 2,transistor 133 is also diode-connected as illustrated in FIG. 7. Thecurrent 126 flowing through transistor 133 does not change with changesin photocurrent 111. The voltage on node N2 is therefore substantiallyfixed. The voltage on node N1, however, does change slightly withchanges in photocurrent 111 as explained above in connection with FIG.2. The currents 125 and 126 and/or the sizes of transistors 129 and 133are chosen such that the voltages on nodes N1 and N2 are identical whenthe photocurrent 111 flowing into input terminal 104 is the desired“trip point input current”. An open-loop voltage comparator 500 comparesthe voltages on nodes N1 and N2. If the voltage on node N1 is higherthan the voltage on node N2 (a high photocurrent condition), thencomparator 500 forces signal DATA to a digital logic high value. If thevoltage on node N1 is lower than the voltage on node N2 (a lowphotocurrent condition), then comparator 500 forces signal DATA to adigital logic low value.

Although certain specific embodiments are described above forinstructional purposes, the teachings of this patent document havegeneral applicability and are not limited to the specific embodimentsdescribed above. Although OLTA 101 of FIG. 2 has a single “trip pointinput current” for detecting both low-to-high input current transitionsand for high-to-low input current transitions, the OLTA in anotherembodiment has a first “trip point input current” for detectinglow-to-high input current transitions and has a second “trip point inputcurrent” for detecting high-to-low input current transitions. Numeroustechniques for providing hysteresis can be employed. For example, thedigital signal on node N4 can be used to switch in an additionalP-channel current mirror that supplies additional current to node N2 (inaddition to current 126) such that the total current supplied to node N2is different depending on whether the voltage on node N4 is high or low.

In one embodiment, the “trip point input current” is adjusted based onthe photocurrent level. For example, where the photocurrent level isdetected to be high, the trip point input current is adjusted to be ahigher trip point input current. Using this higher trip point inputcurrent improves amplifier response time because the parasiticcapacitance of the photodiode need not be discharged as far (whentransitioning from a high photocurrent condition to a low photocurrentcondition) in order for the voltage on node N1 corresponding to the trippoint input current to be reached. The amplifier can therefore detectthe low photocurrent condition more rapidly due to the trip point inputcurrent having been adjusted higher. The OLTA is usable to realize areduced current-consumption sleep mode in which receiving of thebeginning of an infrared signal can wake-up a device such as amicrocontroller from the sleep mode. One way to accomplish this is byselectively reducing currents through selected ones of the P-channelcurrent sources. The OLTA is more sensitive in the sleep mode. Afterwaking up, standard P-channel current source selections would be enabledto receive the remainder of the infrared signal or another infraredsignal. The photodiode and OLTA can be integrated onto the sameintegrated circuit die. Although an OLTA is set forth above where thephotodiode is coupled between VDD and the input of the amplifier,another example of the OLTA involves a photodiode coupled between theinput of the amplifier and ground. In such a case, the N-channeltransistors of the OLTA are replaced with P-channel transistors, and theP-channel transistors of the OLTA are replaced with N-channeltransistors. Accordingly, various modifications, adaptations, andcombinations of various features of the described embodiments can bepracticed without departing from the scope of the invention as set forthin the claims.

1. A microcontroller integrated circuit comprising: an input terminal;and an open-loop transimpedance amplifier (OLTA) having an input nodeand an output node, wherein the input node is coupled to the inputterminal of the microcontroller integrated circuit, the input nodehaving an input impedance less than approximately forty ohms if an inputcurrent of eight milliamperes or more is flowing into the input node,wherein the input impedance decreases as the input current in an inputcurrent operating range increases, wherein the input impedance decreasesmore rapidly than the input current increases such that a voltage on theinput terminal is effectively clamped to an input voltage range of lessthan approximately 0.7 volts for input currents in the input currentoperating range, wherein the OLTA outputs a digital logic signalindicative of whether the input current is in a first low input currentrange or is in a second high input current range.
 2. The microcontrollerintegrated circuit of claim 1, wherein the input current operating rangeextends from zero milliamperes to at least eight milliamperes.
 3. Themicrocontroller integrated circuit of claim 2, wherein the OLTA consumesless than thirty microamperes when receiving an input current signal,wherein the input current signal has a first current level and a secondcurrent level, and wherein the input current signal has a signal ratefrom zero of up to at least five hundred kHz.
 4. The microcontrollerintegrated circuit of claim 1, wherein the OLTA comprises: an N-channeltransistor having a drain, a gate, and a source, wherein the drain iscoupled to the input node of the OLTA, wherein the source is coupled toa ground conductor; and a passgate that is controllable to couple thegate of the N-channel transistor to the drain of the N-channeltransistor.
 5. The microcontroller integrated circuit of claim 4,wherein the OLTA further comprises: a current source that supplies afirst current onto the input node of the OLTA, wherein if no current isflowing into the microcontroller integrated circuit through the inputterminal and if the OLTA is enabled then the current flows from thecurrent source, through the input node, and then from the input node andthrough the N-channel transistor to the ground conductor.
 6. Themicrocontroller integrated circuit of claim 1, wherein the OLTAcomprises: a diode-connected N-channel transistor that biases thevoltage on the input terminal of the OLTA.
 7. The microcontrollerintegrated circuit of claim 1, wherein the OLTA has an input currentsensitivity less than 20 microamperes.
 8. The microcontroller integratedcircuit of claim 1, further comprising: a VDD supply voltage terminal;and a ground terminal, wherein one or more batteries supplies a supplyvoltage onto the VDD supply voltage terminal with respect to a groundpotential on the ground terminal, and wherein an anode of a photodiodeis coupled to the VDD supply voltage terminal, and wherein a cathode ofthe photodiode is coupled to the ground terminal.
 9. The microcontrollerintegrated circuit of claim 1, wherein the OLTA comprises: a currentsource that supplies a first current to the input node of the OLTA; anfirst N-channel transistor that has a drain coupled to the input node ofthe OLTA, a source coupled to a ground conductor, and a gate; a passgatethat is controllable to be conductive or non-conductive, wherein whenthe passgate is conductive the gate of the first N-channel transistor iscoupled to the input node of the OLTA, wherein when the passgate isnon-conductive the gate of the first N-channel transistor is not coupledto the input node of the OLTA; a second N-channel transistor that iscontrollable to couple the gate of the first transistor to ground theground conductor; and a digital processor that can cause a digitalenable signal to have a first digital logic level or a second digitallogic level, wherein if the digital enable signal has the first digitallogic level then the passgate is controlled to be conductive and thesecond N-channel transistor does not couple the gate of the firsttransistor to the ground conductor, whereas if the digital enable signalhas the second digital logic level then the passgate is controlled to benon-conductive and the second N-channel transistor couples the gate ofthe first transistor to the ground conductor.
 10. The microcontrollerintegrated circuit of claim 9, wherein the OLTA further comprises: asecond current source that supplies a second current onto a second node;and a third N-channel transistor that has a drain that is coupled to thesecond node, a source that is coupled to the ground conductor, and agate that is coupled to the gate of the first N-channel transistor. 11.The microcontroller integrated circuit of claim 10, wherein the firstand second current sources are disabled when the digital enable signalhas the second digital logic level.